1. Field of the Invention
This invention relates to a method and apparatus for improving resolution of critical features printed on an optical mask for semiconductor fabrication, and in particular, to a method and apparatus for etching chrome films on optical masks to improve the across mask image size uniformity and linearity and reduce errors in the nominal image size on optical masks.
2. Description of Related Art
In the manufacture of circuit patterns on electronic components, such as the manufacture of integrated circuits on semiconductor substrates, photomasks are used to transfer the desired circuit pattern onto the substrate workpiece. A photomask is typically employed a large number of times for the production of numerous electronic devices. This places stringent demands on the quality of a photomask since any flaws or defects in the photomask are reproduced in the workpiece, which directly affects the operability of the resultant workpiece.
One of the biggest technical challenges in photomask manufacturing is to build a high accuracy photomask that meets the stringent across mask critical dimension uniformity (CDU) specifications required by the semiconductor industry. The across mask CDU is typically obtained by measuring the line width of critical features on the mask at many locations across the mask using a Scanning Electron Microscope (SEM) and is expressed as a 3 sigma value in nanometers. For photomasks used for building semiconductor devices in the 90 nm technology node the mask CDU specification is typically <7 nm (3 sigma). For 65 nm devices the mask CDU specification is typically <5 nm (3 sigma) and for 45 nm devices the mask CDU specification is <3.5 nm (3 sigma). This invention is aimed at enabling achievement of these strict across mask CD uniformity specifications.
A conventional photomask comprises a patterned metal film, such as chrome, deposited onto the surface of a transparent base. To make the photomask the metal film is blanket deposited on the transparent base and covered with a photoresist. A desired pattern is exposed and developed on the photoresist, and then portions of the underlying exposed metal film are removed from the developed areas. Any remaining photoresist is subsequently removed leaving a patterned metal film on the substrate workpiece.
In typical processes of manufacturing a chrome film photomask, a wet etch process may be used in which cerium (IV) ammonium nitrate and perchloric acid are employed. However, the use of a wet etch process in the manufacture of a chrome mask makes it difficult to manufacture the mask with high accuracy as a result of the wet etching processes causing side etching effects/biases.
While the following description will be directed for convenience to chrome-on-glass (COG) masks, other masks such as phase shift (PSM) masks having a phase shift layer of for example MoSi on the glass surface may be made using the method of the invention. In general, the silicide layer is formed over the transparent substrate to provide good joining ability between the mask material of silicide and the substrate. A chrome layer and photoresist layer are then formed on the MoSi layer as described above.
Dry etch processes are preferably employed for the formation of chrome photomasks. A typical dry etch process of chrome masking typically employs the use of a mixed gas of chlorine, e.g., Cl2 and oxygen (O2). The dry etch processes were found to be advantageous for manufacturing a mask with high accuracy, however, the etch rate of chrome using conventional dry etch processes is low and not selective. Additionally, such conventional dry etch processes typically cause several defects in the pattern for the photomask, such as opaque defects, clear defects, and poor resolution features, which in turn are transferred to the workpiece rendering it inoperable.
As a result of the difference in etch selectivity between the resist and chrome films, the resists have been modified to improve their etch resistance and selectivity to the underlying chrome film during dry etch processes. For example, a prior art technique to improve resolution features on the resultant mask is to reduce the thickness of the photoresist film deposited thereover the chrome film. The thinning of the resist film may however, lead to the resist film being completely removed during the chrome etch before the patterned images are fully transferred into the chrome layer. Other techniques of modifying the resists to improve their etch resistance and selectivity to the underlying chrome film during dry etch processes has been to modify the resist chemistry. However, it has been found that increasing etch resistance of the material can also degrade important resist performance parameters, such as, sensitivity, image quality and stability of the resist.
Accordingly, a need continues to exist in the art to provide an improved method to make a photomask that allows for the achievement of the desired nominal image size and image size uniformity on the photomask with a method which is efficient, easy and relatively inexpensive.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved method for manufacturing a photomask such as a COG mask and a PSM mask which achieves a desired minimum feature size resolution on a photomask.
It is yet another object of the present invention to provide a method of making a photomask such as a COG mask and a PSM mask that will have improved across mask critical dimension uniformity and linearity of the photomask.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.